He (breath) is also Brihaspati, for speech is Brihati (Rig-veda), and he is her lord; therefore he is Brihaspati.

In plain words: Breath is also called Brihaspati, a name meaning "lord of speech." Speech is called Brihati, a name for the Rig Veda's verses, and breath is its master, since we need breath to speak.